1. Field of the Invention
The invention relates to a filter device for changing a sample rate of a discrete representation, e.g., to change a size of an image signal, and to an image display apparatus comprising such a filter device.
2. Description of the Related Art
With the advent of computer technology in the consumer electronics area by means of, e.g., internet, games and video conferencing, it is likely that the display technologies of television and computers are going to merge. As a result of this emerging trend, the desire for improved quality in TV images will increase, because the spatial resolution of PC-based synthetic images outperforms those of natural video scenery. Furthermore, in these state-of-the-art consumer electronics products, a need for high quality image scalers arises. Until now, sampling rate converters were more or less dedicated and optimized to perform one function, e.g., size conversion for Picture In-Picture (PIP). This leads to products in which sampling rate converters, tuned for different applications, are scattered. We foresee a need for a more flexible high quality scaler that can be used for a range of functions, not only to allow new advanced scaling features, but also to be able to handle different picture formats and display types. One could think of PIP, split-screen, multi-window features, high quality size conversion of graphics and video, and size adaptation for LCD or plasma displays.
U.S. Pat. No. 5,383,145 discloses direct type and inverted type digital FIR filters. When such FIR filters are used for compression, i.e., for decreasing the size of an image signal, too many samples are obtained as such a FIR filter calculates one output sample for each input sample. This means that at the output, some of the output samples have to be discarded, which implies a waste of calculation resources used to obtain these discarded samples. Moreover, there is no indication as to how an expansion is to be effectuated.